Two types of substances for cultivation of agricultural crops are known: "active" which gives nutritious substances to the the plants and "inert" in which the nutrition of the plants is provided for by a nutritious solution.
An "active" type of substrate is known which consists of soil, peat and sand, mixed in different proportions and most frequently in proportions 1:2:1. It is the basic substrate used for the cultivation of strawberries and other crops in cultivating equipment.
For planting of green cuttings several kinds of substrates are known, consisting of peat, sand, perlite and vermiculite. Each of them is used independently or in combination with the rest of the above mentioned materials.
For the growing of strawberries in greenhouses an "inert" substrate was tried, this substrate comprising peat, moss and sand (recently vermiculte as well).
In practice on areas of considerable size an "inert" substrate is used and can comprise peat, perlite, wood filings, granules of cork oak and other light materials.
Plant nutrition in "inert" substances is carried out using the principle of hydroponics, i.e. through supply with nutritious solutions, prepared in advance. Of the so called "active" substrates one has been tried out in Bulgaria. Its constituents are soil, peat and sand in proportion 1:1:1.
Peat, sand and perlite are known to be used as substrates for the rooting of green cuttings. These materials are used either separately or in mixtures of peat and sand or peat and perlite in different volume ratios, e.g. 1:1, 2:1, 3:1, 1:2. In some countries sand, peat and vermiculite, pure or in mixtures, are used for the planting of green cuttings.
The disadvantage of the "active" substrates used in current practice is that they contain in small quantities some macro and microelements, easily accessible to the plants and yield them with difficulty. Besides they have high bulk weight and low porosity, the aeration of the medium is not very good; the nutritious substances accessible to the plants are not in quantities sufficient to provide for a high yield; the pH with the medium depends too much on the type of soil of which the substrate is combined. Such substrates are frequently infected with diseases and pests.
The disadvantage that "active" substrates contain small quantities of macro- and microelements which are yielded with difficulty is of peculiar importance when plants are rooted under artifical mist. Under these conditions some of the macro- and microelements are to a great extent washed from the leaves of the cuttings. As the plants cannot get them easily from the substrate, the results of the planting are not always good. The insufficient quantity of nutritious substances in the rooted plants predetermines the low percentage of rooting in the greenhouse.
The substances used in practice have low sorption ability and the nitrogenous, phosphorous and potassium fertilizers introduced into them are intensively washed away by the filtrating waters outside the rooting zone.
"Inert" substrates, though lighter and rather practical at first consideration, have several disadvantages as well: They require preliminary preparation and control over the concentration of the nutritious solution, expensive electronic equipment and highly qualified staff for the preparation and dosage of the solutions in the plant pots.